Radiation mask adapted to be aligned with a photoresist layer and method of making the same

ABSTRACT

The present invention provides a procedure for achieving accurate alignment between an X-ray mask and a device substrate for the fabrication of multi-layer microstructures. A first photoresist layer on the substrate is patterned by a first X-ray mask to include first alignment holes along with a first layer microstructure pattern. Mask photoresist layers are attached to second and subsequent masks that are used to pattern additional photoresist layers attached to the microstructure device substrate. The mask photoresist layers are patterned to include mask alignment holes that correspond in geometry to the first alignment holes in the first photoresist layer on the device substrate. Alignment between a second mask and the first photoresist layer is achieved by assembly of the second mask onto the first photoresist layer using alignment posts placed in the first alignment holes in the first photoresist layer that penetrate into the mask alignment holes in the mask photoresist layers. The alignment procedure is particularly applicable to the fabrication of multi-layer metal microstructures using deep X-ray lithography and electroplating. The alignment procedure may be extended to multiple photoresist layers and larger device heights using spacer photoresist sheets between subsequent masks and the first photoresist layer that are joined together using alignment posts.

This invention was made with United States government support awarded bythe following agencies: DOD ARPA Grant No. N00014-93-1-0911 and NSFGrant No. ECS9116566. The United States has certain rights in thisinvention.

This application is a divisional of application Ser. No. 08/753,645,filed Nov. 27, 1996, now abandoned.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor andmicromechanical devices and processing techniques therefor, andparticularly to the formation of multilevel microminiature structuressuch as those formed of metal.

BACKGROUND OF THE INVENTION

Deep X-ray lithography involves a substrate which is covered by thickphotoresist, typically several hundred microns in thickness, which isexposed through a mask by X-rays. X-ray photons are much more energeticthan optical photons, which makes complete exposure of thick photoresistfilms feasible and practical. Furthermore, since X-ray photons are shortwavelength particles, diffraction effects which typically limit devicedimensions to two or three wavelengths of the exposing radiation areabsent for mask dimensions above 0.1 micron. If one adds to this thefact that X-ray photons are absorbed by atomic processes, standing waveproblems, which typically limit exposure of thick photoresist by opticalmeans, become a non-issue for X-ray exposures. The use of a synchrotronfor the X-ray source yields high flux densities, several watts persquare centimeter, combined with excellent collimation to produce thickphotoresist exposures with minimal horizontal run-out. Locally exposedpatterns should therefore produce vertical photoresist walls if adeveloping system with very high selectivity between exposed andunexposed photoresist is available. This requirement is satisfied bypolymethylmethacrylate (PMMA) as the X-ray photoresist, and an aqueousdeveloping system. See, e.g., H. Guckel, et al., "Deep X-ray and UVLithographies for Micromechanics", Technical Digest, Solid State Sensorand Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp. 118-122.

As the thickness of the photoresist is increased, e.g., beyond 500 μm,difficulties have been encountered when relatively soft (lower photonenergy) X-ray sources are used for photoresist exposure. The wavelengthsof soft X-rays are long enough that diffraction effects can lead topenumbral blurring of the X-ray mask shadow image. Difficulties can alsoarise from interface phenomena caused by the non-negligible reflectivityof common photoresist materials with respect to soft X-rays. It has alsobeen found that for resist sheets thicker than about 500 μm, soft X-rayscannot penetrate sufficiently to the bottom of the resist layer toguarantee development of the bottom regions of the resist without such alarge exposure that the dose at the upper regions of the resist wouldbecome so high as to cause destructive overexposure. Such overexposurecan cause distortion or fracture of the resist layer, or evendestruction of the fragile soft X-ray mask which is typically positionedwithin a few microns of the resist layer. However, these problems can beavoided by exposing thick photoresist layers using hard (high energy)X-rays. Hard X-rays may be obtained by spectral shaping the X-ray beamfrom a source, such as a synchrotron, having a significant spectralcontent of high energy X-rays to substantially eliminate lower energyphotons which have a short absorption length in the photoresist, andwhich would tend to be substantially absorbed near the top surface ofthe photoresist. Spectral shaping for hard X-rays may be accomplished byspectral filtering using, e.g., aluminum and/or silicon filters. Thepenetrating ability of hard X-rays allows photoresist layers up toseveral centimeters thick to be exposed, allows multiple stackedphotoresist layers and substrates to be exposed simultaneously, andallows the X-ray mask to be formed on robust substrates, such asrelatively thick and wide single crystal silicon wafers, on which anX-ray absorber such as gold is deposited in a pattern.

Deep X-ray lithography may be combined with electroplating to form highaspect ratio structures. This requires that the substrate be furnishedwith a suitable plating base prior to photoresist application.Typically, this involves a sputtered film of adhesive metal such aschromium or titanium, which is followed by a thin film of a metal whichis to be plated. Exposure through a suitable mask and development arefollowed by electroplating. This results, after clean-up, in fullyattached metal structures with very high aspect ratios. Such structureswere reported by W. Ehrfeld and co-workers at the Institute for NuclearPhysics at the University of Karlsruhe in West Germany. Ehrfeld termedthe process "LIGA" based on the first letters of the German words forlithography and electroplating. A general review of the LIGA process isgiven in the article by W. Ehrfeld, et al., "LIGA Process: SensorConstruction Techniques Via X-ray Lithography", Technical Digest, IEEESolid State Sensor and Actuator Workshop, 1988, pp. 1-4.

The addition of a sacrificial layer to the LIGA process facilitates thefabrication of fully attached, partially attached, or completely freemetal structures. Because device thicknesses are typically larger than10 microns and smaller than 300 microns, free standing structures willnot distort geometrically if reasonable strain control for the platedfilm is achieved. This fact makes assembly in micromechanics possible,and thereby leads to nearly arbitrary three-dimensional structures. SeeH. Guckel, et al., "Fabrication of Assembled Micromechanical ComponentsVia Deep X-ray Lithography", Proceedings of IEEE Micro ElectroMechanical Systems, Jan. 30-Feb. 2, 1991, pp. 74-79.

It is possible to extend the LIGA process, with or without a sacrificiallayer, for the formation of multi-layer micromechanical structures. Thisis achieved by performing several X-ray exposures of multiplephotoresist layers, with electroplating of additional layers of metalafter each exposure. In such a procedure, metal microstructures may beelectroplated both onto the substrate plating base and onto the topsurface of previously deposited microstructure layers. Replanarizationof previously deposited microstructure layers, to achieve asubstantially flat uniform surface, is typically required beforeelectroplating subsequent metal microstructure layers thereon. Variousmachining, lapping, and polishing procedures may be used forreplanarization.

A key to successful fabrication of multi-layer micromechanical devicestructures is the accurate alignment of subsequently depositedmicrostructure layers with previously deposited ones. For example, if agear-shaft-gear combination is to be fabricated, with each component ofthe combination formed on a different microstructure layer, only a smalleccentricity between the axes of the gears and shaft, and thus only asmall alignment error between the microstructure layers, can betolerated. Hence, a procedure to achieve high alignment accuracy betweenlayers must be available in order to extend LIGA for the fabrication ofmulti-layer micromechanical devices.

Alignment procedures similarly play an important role in the relatedfield of microelectronics fabrication. Microelectronics manufacturinguses alignment procedures for the fabrication of multi-level integratedcircuits. Alignment procedures used in Very Large Scale Integration(VLSI) processing for microelectronics, for example, are usually of anoptical nature. In the VLSI process, each level is formed to containreference marks at a certain location. These marks are used foroptically aligning a subsequent mask to a previous level. A microscopeis focused on the alignment marks on both the previous work level andthe subsequent level mask. The mask is then positioned such that themarks on the work level and the mask are optically aligned with eachother, thus aligning the subsequent mask pattern with the previous levelmicroelectronic pattern. During optical exposure through the subsequentmask, new alignment marks are transferred to the current work level.Using this method, the several layers which make up an integratedcircuit are aligned with each other.

Since visible light is used for the optical alignment procedure, theprocedure is limited to alignment gaps between the substrate and themask which lie in the range of the depth of focus of the alignmentmicroscope. In VLSI processing, the alignment gap between the devicesubstrate and the mask is on the order of a few micrometers. However, inLIGA processes for the fabrication of relatively thick metalmicrostructures, the mask and the device substrate may typically be morethan 100 micrometers apart. For the magnification required to opticallyalign the LIGA substrate to a mask with submicron tolerance, the depthof focus is much less than 100 micrometers. Moreover, X-ray masks usedin LIGA processing are typically formed from a silicon wafer (for hardX-rays) or a silicon wafer which includes a silicon nitride membrane(for soft X-rays) upon which a thin metal plating base layer isdeposited for subsequent formation of the mask pattern by plating of anX-ray blocking material, such as gold, onto the plating base layer. TheX-ray masks used in LIGA processing in most cases are, therefore, notoptically transparent. For these reasons, the optical alignmentprocedure used in VLSI processing cannot easily be applied to thefabrication of multilayer micromechanical devices by the LIGA process.

An alignment procedure that may be used for fabrication of multi-layermicrostructures by the LIGA process is suggested in U.S. Pat. No.5,378,583, to Guckel. et al., entitled "Formation of MicrostructuresUsing a Preformed Photoresist Sheet". This patent describes the use ofthick preformed sheets of photoresist for the fabrication of photoresistand metal microstructures. Multiple photoresist sheets may be exposed toX-rays in various patterns, and then adhered together into a laminate.The laminate may then be adhered to a substrate and used for thefabrication of metal microstructures. Alignment of the variousphotoresist layers, such that microstructure regions formed therein byexposure to X-rays are aligned with each other, may be achieved bycreating mechanical alignment structures during exposure of eachphotoresist layer, and then using these alignment structures to obtainmechanical registration between a previous layer and a subsequent layer.Exemplary alignment structures may consist of relatively large holesformed in each photoresist layer on opposite sides of the patternedportion thereof. The holes may be formed as part of the X-ray exposureand developing process for patterning the photoresist layer, so that themicrostructure pattern and the alignment holes are formed simultaneouslyfor each layer, and so that the relative position of the alignment holeswith respect to the microstructure pattern is precisely controlled. Thealignment holes are designed to accept pegs, which may be formed of aphotoresist material or metal. Alignment between photoresist layers isachieved by assembling mounting holes in a subsequent layer onto thepegs placed in mounting holes from the previous layer, and gluing thesubsequent photoresist layer to the underlying layer. Self-alignment isobtained because the alignment holes are exposed at the same time as thedesired pattern in each layer, and therefore the alignment tolerance isgoverned by assembly tolerances. After the desired number of photoresistlayers have been built up into a laminate, metal may be electroplatedinto the microstructure pattern regions in the photoresist laminate, ifdesired, to effectively form a multi-layer metal microstructure.

SUMMARY OF THE INVENTION

The present invention provides an alignment by assembly procedurewhereby an X-ray mask may be aligned with a previously patternedphotoresist layer such that a microstructure pattern defined by thesubsequent mask is precisely aligned with the previously producedmicrostructure pattern. Precise alignment is achieved by direct assemblyof an X-ray mask upon a previously processed first photoresist layer. Afirst X-ray mask is used to form a microstructure pattern and firstalignment holes in the first photoresist layer. Corresponding maskalignment holes are formed in a mask photoresist layer attached to thesubsequent X-ray mask. Alignment of the microstructure pattern definedby the subsequent X-ray mask and the microstructure pattern previouslyformed in the first photoresist layer is achieved by assembling thesubsequent X-ray mask onto the first photoresist layer using alignmentposts mounted in the first alignment holes in the first photoresistlayer and in the mask alignment holes in the mask photoresist layer onthe subsequent X-ray mask. The alignment procedure of the presentinvention is thus entirely mechanical, and does not require the use ofcomplicated alignment equipment, such as an optical alignmentmicroscope.

The alignment procedure of the present invention is particularlyapplicable to the fabrication of multi-layer microstructures usingmultiple photoresist layers. The alignment procedure of the presentinvention may be used for the fabrication of multi-layer photoresiststructures, multi-layer photoresist laminates including microstructuresformed therein, and multi-layer metal microstructures formed by the LIGAprocess using multiple photoresist layers. The alignment procedure ofthe present invention may also be used in the fabrication of singlelayer microstructures, to align the microstructure patterns defined bymultiple X-ray masks onto a single photoresist layer. The presentinvention may be used with X-ray masks employed for photoresist layerexposure using either soft or hard X-rays.

The alignment procedure of the present invention begins with theapplication of a first thick photoresist layer, up to several hundredmicrons in thickness, unto a substrate. For the fabrication of amulti-layer metal microstructure using the LIGA fabrication process, thesubstrate preferably has a metal plating base layer formed thereonbeneath the first photoresist layer. The first layer of photoresist maypreferably be formed as a preformed photoresist sheet which is bonded tothe plating base layer. The first photoresist layer is exposed to X-raysthrough a first X-ray mask to define a pattern in the first photoresistlayer. The pattern defines the first layer microstructure pattern andfirst alignment hole geometries to be formed in the first photoresistlayer. For soft X-ray exposure, the mask may be formed on a siliconwafer substrate which has thin silicon nitride device and alignmentstructure membrane areas formed thereon. Multiple alignment structuremembrane areas are preferably defined, one for each alignment hole to beformed in the first photoresist layer on the device substrate. Thealignment structure membrane areas are preferably positioned on oppositesides of the device membrane area to minimize the effect of alignmentstructure errors on the microstructure devices to be formed. A maskpattern of an X-ray absorbing material, such as gold, is then formedover the device and alignment structure membrane areas. The mask patterndefines, in the device membrane area, the first layer metalmicrostructures to be produced, and, in the alignment structure membraneareas, alignment hole geometries to be formed in the first photoresistlayer. The alignment hole geometry is preferably a square or rectangularshape with rounded corners, although other shapes may also be used. Forhard X-ray exposure, the X-ray mask may be formed on a robust substrate,such as relatively thick and wide single crystal silicon wafers. Sincethe hard X-rays are able to penetrate the X-ray mask substrate, the hardX-ray mask need not have the membrane areas formed thereon, rather, theX-ray mask pattern is formed by deposition of X-ray absorbing materialsdefining the device pattern and alignment hole geometries on device andalignment structure areas of the hard X-ray mask, respectively.

The first layer microstructure pattern and alignment hole geometries aretransferred to the first photoresist layer on the device substrate byexposing the first photoresist layer to X-rays through the first X-raymask. Exposed portions of the first photoresist layer thus correspond tothe microstructure device pattern and the alignment hole geometriesdefined by the first X-ray mask pattern. These exposed portions of thefirst photoresist layer are removed by a developing process to formfirst alignment holes in the first photoresist layer, and, for metalmicrostructure fabrication, to expose the plating base layer in thepattern of the first microstructure layer to be formed. A first layer ofa multi-layer metal microstructure may then be deposited byelectroplating onto the plating base, into the areas of the firstphotoresist layer corresponding to the microstructure device patternthat have been removed by the developing step. The first photoresistlayer and the first layer microstructures formed therein may then beplanarized and polished in preparation for the application of a secondphotoresist layer onto the first photoresist layer, and deposition ofsubsequent metal layers onto the previously deposited layer.

A second photoresist layer is then applied over the first photoresistlayer. The second photoresist layer is preferably a preformedphotoresist sheet which is bonded to the first photoresist layer over aportion of the first photoresist layer which, for metal microstructuredevice fabrication, includes the deposited first layer metalmicrostructures. The second photoresist layer does not cover the firstalignment holes formed in the first photoresist layer.

A second X-ray mask, for either soft or hard X-ray exposures, is formedin the manner previously described with respect to the first X-ray mask.The second X-ray mask includes a device area patterned with X-rayabsorbing material in the desired pattern of, for example, a secondlayer of metal microstructure devices to be formed. Alignment structureareas of the second X-ray mask have X-ray absorbing materials formedthereon in a pattern corresponding to the geometry of the firstalignment holes formed in the first photoresist layer on the devicesubstrate. The alignment structure and device patterns of the first maskand the second mask are laid out such that when the alignment structuresdefined by each mask are aligned, the device patterns are also aligned.Thus, the microstructure device patterns formed on the X-ray masks, andsubsequently transferred to photoresist layers on the device substrate,will be in proper alignment when the alignment structure patterns on theX-ray masks and in the photoresist layers are aligned.

A mask photoresist layer is applied over each alignment structure areaof the second X-ray mask. The photoresist layer may be applied over thealignment structure area on the front side of the X-ray mask, directlyover the alignment hole geometry mask pattern, or over the back of theX-ray mask in an area corresponding to the alignment structure area onthe front of the mask. Preferably, the mask photoresist layer is appliedto the front side of the X-ray mask over the alignment structure area,but elevated therefrom. This is accomplished by first applyingphotoresist sheets to the front side of the X-ray mask outside of thealignment structure area, and then attaching the mask photoresist layerto these photoresist sheets over the alignment structure area. The maskphotoresist layer thereby extends between the photoresist sheets overthe alignment structure area, e.g., over the alignment structuremembrane for a soft X-ray mask. The mask photoresist layer is thenexposed to X-rays through the second X-ray mask to transfer thealignment hole geometry defined by the second X-ray mask pattern ontothe mask photoresist layer. The exposed mask photoresist layer is thendeveloped to form mask alignment holes therein, corresponding in shapeto the alignment hole geometry defined by the second X-ray mask patternand the first alignment holes formed in the first photoresist layer onthe device substrate.

The second X-ray mask is assembled onto the first photoresist layerusing alignment posts. The alignment posts have a geometry correspondingto that of the alignment holes, and are sized to fit snugly therein. Thealignment posts may be formed of metal by a LIGA fabrication processemploying a sacrificial layer to form freed metal parts. Alternatively,and preferably, the alignment posts may be formed of a photoresistmaterial. This may be accomplished by attaching a thick preformedphotoresist sheet to a sacrificial layer formed on a substrate. Thephotoresist layer is then exposed to X-rays through a mask whichincludes X-ray absorbers formed thereon in a pattern defining the shapeof the alignment posts. The photoresist layer is then developed toremove the exposed portions thereof, leaving behind freestandingphotoresist structures in the shape of the alignment posts. Thesacrificial layer is then removed from beneath the freestandingphotoresist structures to free the alignment posts from the substrate.

The alignment posts are used to assemble the second X-ray mask onto thefirst photoresist layer. Alignment posts are placed in the firstalignment holes formed in the first photoresist layer on the devicesubstrate such that the alignment posts extend above the surface of thefirst photoresist layer. The second X-ray mask is then positioned overthe first photoresist layer until the alignment posts penetrate into themask alignment holes on the second X-ray mask. The alignment posts thusautomatically ensure proper alignment of the first alignment holesformed in the first photoresist layer of the device substrate with themask alignment holes formed in the photoresist layer on the second X-raymask. This also automatically aligns the second X-ray maskmicrostructure device pattern with the microstructure device patternpreviously formed in the first photoresist layer.

The second photoresist layer on the device substrate may then be exposedto X-rays through the second X-ray mask to transfer the second layermicrostructure device pattern to the second photoresist layer. Thesecond photoresist layer is then developed to remove the exposed portionthereof and to form the second layer microstructure device patterntherein. A second layer of metal microstructure devices may be depositedinto the pattern formed in the second photoresist layer byelectroplating onto the tops of the first layer microstructure devicesformed in the first photoresist layer. Second layer microstructuredevices may also be electroplated directly onto the plating base layerif the exposure of the second photoresist layer through the second X-raymask is allowed to proceed for a sufficient duration to also expose thefirst photoresist layer through to the plating base layer.

The alignment procedure of the present invention may be extended forsubsequent photoresist layers applied on top of the second photoresistlayer. Metal microstructures of more than two layers may thus be formedby aligning subsequent X-ray masks with the first photoresist layer onthe device substrate. As with the second X-ray mask, each subsequentX-ray mask includes mask alignment holes formed in a mask photoresistlayer attached thereto. These mask alignment holes are joined to thefirst alignment holes in the first photoresist layer using longeralignment posts, or multiple alignment posts stacked in series incombination with spacer sheets made out of photoresist and havingcentral apertures shaped to receive the alignment posts. Spacer sheetsmay also be used generally to ensure a sufficiently large exposure gapbetween the second X-ray mask and the second photoresist layer.

The alignment procedure of the present invention may be used for thefabrication of multi-layer metal microstructures attached to asubstrate. Free multi-layer metal microstructure parts may also befabricated using the present invention if a sacrificial layer isemployed between the metal microstructures and the substrate. Since thealignment procedure of the present invention does not interfere withdevice substrate preparation, the present invention is fully compatiblewith IC fabrication techniques, and may be used for the fabrication ofmulti-layer metal microstructures on substrates containing preformedintegrated circuits. The alignment procedure of the present inventionalso does not significantly reduce the substrate area available formicrostructure device fabrication.

The alignment procedure of the present invention is cost-effectivebecause conventional LIGA processing tools, which are already available,are employed in fabricating the alignment structures. The alignmentprocedure of the present invention is an inexpensive and time efficientprocess. Two microstructure levels may be aligned in five to ten minutesat the X-ray exposure site, without requiring any expensive optical orprecision alignment instruments.

The alignment procedure of the present invention can accommodatedifferent photoresist sheet heights and gaps between the X-ray mask andthe device substrate. The ability to accommodate more than twophotoresist layers on the substrate also exists.

The alignment procedure of the present invention may be standardized forrepetitive use in multi-level LIGA processes. Standardized X-ray maskshapes, alignment structure geometries, and processing sequences, can beused.

The alignment procedure can be used for a variety of different X-raybeam, X-ray mask, and wafer sizes. A simple re-design of device andalignment membranes can accommodate virtually any wafer size.

The alignment procedure of the present invention has been employed toachieve alignment accuracies of better than 4 micrometers for LIGAstructures greater than 100 micrometers in height. Such a range ofaccuracy is close to optimum for precision fabrication using LIGA-likeprocessing.

The alignment procedure of the present invention may be used for thefabrication of microstructure devices using hard X-ray exposures. Thisallows very thick (up to several cm) photoresist layers to be exposed.Moreover, since hard X-rays can penetrate the substrate upon which thephotoresist layers are attached, a hard X-ray mask aligned with thesubstrate in accordance with the present invention on one side of thesubstrate may be used to expose multiple photoresist layers on theopposite side of the substrate.

It is apparent that the alignment procedure of the present invention maybe employed in the fabrication of non-metal microstructures, such asphotoresist microstructures and photoresist laminations havingmicrostructures formed therein. The alignment procedure may also beemployed with radiation masks designed for use with other than X-rayradiation, such as UV radiation masks used in integrated circuitfabrication.

Further objects, features, and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a simplified illustrative side view of a substrate with aplating base and a first photoresist layer formed thereon.

FIG. 2 is a simplified illustrative side view of an X-ray mask waferhaving a silicon nitride layer formed thereon.

FIG. 3 is a view of the X-ray mask wafer of FIG. 2 with a layer ofphotoresist formed on the back side thereof.

FIG. 4 is an illustrative view showing the X-ray mask wafer of FIG. 3exposed through a mask to UV light.

FIG. 5 is an illustrative plan view of the X-ray mask wafer of FIG. 4showing a layout of device and alignment structure membrane areasthereon.

FIG. 6 is an illustrative plan view of an X-ray mask wafer showing analternative layout of device and alignment structure membrane areasthereon.

FIG. 7 is an illustrative side view showing the X-ray mask wafer of FIG.4 after development of the exposed photoresist layer.

FIG. 8 is an illustrative view showing the X-ray mask wafer of FIG. 7after etching of a portion of the silicon nitride layer from the backside of the mask wafer to form silicon nitride windows in the areas fromwhich the exposed photoresist was removed during development, and afterremoval of the remaining photoresist layer.

FIG. 9 is an illustrative side view of the X-ray mask wafer of FIG. 8after etching of the X-ray mask wafer from the back side in the siliconnitride windows to form device and alignment structure membranesthereon.

FIG. 10 is an illustrative side view of the X-ray mask wafer of FIG. 9with the addition of plating base and photoresist layers formed on thefront side thereof.

FIG. 11 is an illustrative view showing the X-ray mask wafer of FIG. 10exposed through a master mask to UV light.

FIG. 12 illustrates the preferred shape of an alignment hole geometryformed in accordance with the method of the present invention.

FIG. 13 illustrates an alternative alignment hole geometry formed inaccordance with the method of the present invention.

FIG. 14 is an illustrative view showing the X-ray mask wafer of FIG. 11after development of the exposed photoresist layer formed on the frontside thereof.

FIG. 15 is an illustrative view showing the X-ray mask wafer of FIG. 14after an X-ray absorbing material has been electroplated onto theplating base layer into areas of the photoresist layer which wereremoved by development, and after removal of the remaining photoresistlayer.

FIG. 16 is an illustrative view showing the substrate of FIG. 1 exposedthrough the X-ray mask of FIG. 15 to X-ray radiation.

FIG. 17 is an illustrative side view of the exposed substrate of FIG. 16showing masking tape placed over exposed portions of the firstphotoresist layer corresponding to first alignment holes.

FIG. 18 is an illustrative plan view of the substrate of FIG. 17.

FIG. 19 is an illustrative side view of the substrate of FIG. 17 afterthe first photoresist layer has been developed to expose a portion ofthe plating base layer in a device area of the substrate.

FIG. 20 is an illustrative side view of the substrate of FIG. 19 withmetal microstructures electroplated onto the plating base layer intoareas of the first photoresist layer which were removed duringdevelopment.

FIG. 21 is an illustrative side view of the substrate of FIG. 20 afterthe photoresist layer is developed for a second time to form firstalignment holes therein.

FIG. 22 is an illustrative side view of the substrate of FIG. 21 with asecond photoresist layer formed thereon over the microstructures formedin the first photoresist layer.

FIG. 23 is an illustrative side view of a second X-ray mask with maskphotoresist layers attached directly over alignment structure membraneareas formed on the front side thereof.

FIG. 24 is an illustrative side view of an alternative embodiment secondX-ray mask with mask photoresist layers attached to the back sidethereof over areas corresponding to the alignment structure membraneareas of the X-ray mask.

FIG. 25 is an illustrative side view of a second X-ray mask with maskphotoresist layers attached to the front side thereof elevated over thealignment structure membrane areas.

FIG. 26 is an illustrative plan view of the second X-ray mask of FIG.25.

FIG. 27 is an illustrative side view of the second X-ray mask of FIG. 25after the mask photoresist layers attached thereto have been exposed toX-ray radiation through the X-ray mask and developed to form maskalignment holes in the mask photoresist layers.

FIG. 28 is a simplified illustrative side view of a substrate with asacrificial layer formed thereon and a photoresist sheet adhered to thesacrificial layer.

FIG. 29 is an illustrative view showing the photoresist and substrate ofFIG. 28 exposed to X-ray radiation through an X-ray mask patterndefining alignment post geometries.

FIG. 30 is an illustrative view showing the substrate and photoresist ofFIG. 29 after the exposed photoresist has been developed to form freestanding alignment posts attached to the substrate.

FIG. 31 is a perspective view of an alignment post for use in accordancewith the present invention.

FIG. 32 is an illustrative side view of an assembly of the second X-raymask of FIG. 27 aligned with the substrate of FIG. 22 using alignmentposts in accordance with the present invention, with the substratephotoresist layers exposed to X-ray radiation through the aligned secondX-ray mask.

FIG. 33 is a detailed illustrative side view of an alignment postjoining together mask alignment holes in a second X-ray mask and firstalignment holes in a first photoresist layer on a substrate.

FIG. 34 is an illustrative side view of an assembly of the second X-raymask of FIG. 27 aligned with the substrate of FIG. 22 using spacersheets and multiple stacked alignment posts in accordance with thepresent invention, with the photoresist layers on the substrate exposedto X-ray radiation through the second X-ray mask.

FIG. 35 is an illustrative side view of the substrate of FIG. 32 or 34after the exposed photoresist layers have been developed.

FIG. 36 is an illustrative view of the substrate of FIG. 35 withmulti-layer microstructures formed thereon in portions of thephotoresist layers removed during developing, and after remainingportions of the photoresist layers have been removed.

DETAILED DESCRIPTION OF THE INVENTION

The alignment procedure of the present invention will be described indetail with respect to the exemplary application of the fabrication of atwo-level metal microstructure using the LIGA fabrication process. Thedetails of LIGA processing itself are know to those having skill in theart. A detailed description of an exemplary process for carrying out theproduction of micromechanical structures using the LIGA process isdescribed, for example, in U.S. Pat. No. 5,190,637, to Henry Guckel,entitled "Formation of Microstructures by Multiple Level Deep X-rayLithography with Sacrificial Metal Layers," the disclosure of which isincorporated herein by reference. It should be noted that the alignmentprocedure of the present invention may also be applied to othermicromechanical and microelectronic device fabrication processes. Inparticular, the alignment procedure of the present invention may beemployed for the fabrication of multi-layer photoresist microstructures,multi-layer photoresist laminates including microstructures formedtherein, and single layer photoresist and metal microstructuresfabricated using multiple exposures through different X-ray masks.Variations on the exemplary procedure described will be discussedthroughout this detailed description.

With reference to FIG. 1, a device substrate 40 is provided, upon whicha multi-layer metal microstructure will be formed. The substrate 40 maycomprise a variety of materials, including semiconductors, insulators,and so forth. The substrate 40 may also incorporate preformed integratedcircuit structures. Since the alignment procedure of the presentinvention does not interfere with preparation of the substrate 40, theprocedure allows the formation of aligned multi-layer microstructures incombination with electronics in the substrate 40. Typically, a metalplating base layer 41 will be deposited onto the top surface of thesubstrate 40, such as by sputtering. The substrate 40 could be composedof a conductive metal, in which case no plating base may be necessary.For the formation of parts to be freed from the substrate 40, asacrificial release layer (not shown) may be used beneath the platingbase layer 41. A first photoresist layer 43, for example,polymethylmethacrylate (PMMA) is then applied to the substrate.Depending on the type of radiation used to expose the photoresist 43,the depth of exposure desired, and the properties of availablephotoresist, it is apparent that positive or negative photoresist may beused. Throughout the remainder of this description, a positivephotoresist which becomes susceptible to removal by a developer uponexposure to X-rays will be assumed. It is understood, however, thatother types of photoresist, including negative photoresist materials, aswell as photoresist sensitive to other types of radiation, including UVradiation, may be employed to form microstructures using the alignmentprocedure of the present invention. The first photoresist layer 43 maybe cast-on to the desired thickness, or may be applied as a pre-formedphotoresist sheet. For thicker photoresist layers, especially thosehaving thicknesses greater than 200 microns, use of a preformedphotoresist sheet is preferred. The curing process applied to a cast-onphotoresist layer of such a thickness can cause internal strain in thephotoresist layer that can distort the cast-on photoresist layer, andcause loss of adhesion between the photoresist layer 43 and thesubstrate 40. The preformed photoresist sheet 43 is preferably adheredto the surface of the substrate 40 using a suitable adhesive, such as arelatively thin spun-on film of PMMA which is applied to the substrate40 in a conventional manner, and cured to form a strong bond to thesubstrate 40. The use of preformed photoresist sheets in the formationof metal microstructures is described in more detail in U.S. Pat. No.5,378,583, to Henry Guckel, et al., entitled "Formation ofMicrostructures Using A Preformed Photoresist Sheet", the disclosure ofwhich is incorporated herein by reference.

The next step in the LIGA fabrication process is to expose the firstphotoresist layer 43 to X-rays through an X-ray mask, to transfer amicrostructure device pattern from the mask to the first photoresistlayer 43. In accordance with the present invention, a first alignmenthole pattern is simultaneously transferred from the X-ray mask to thefirst photoresist layer 43. An exemplary process for fabricating anX-ray mask to provide both microstructure device and alignment holepatterns is described with reference to FIGS. 2-15. This example assumesexposure of the photoresist layer 43 using soft X-rays. A mask for usewith soft X-rays has thin membrane areas formed on a thick substrate.The substrate provides the structural support for the mask, with themembrane areas defining those areas of the mask through which softX-rays may pass. For thicker photoresist layers, e.g., up to severalcentimeters in thickness, the use of high energy hard X-rays ispreferred. Since hard X-rays can penetrate mask substrate materials, theuse of thin and fragile membrane areas is not required. Instead, a hardX-ray mask may be formed by depositing the X-ray absorbing mask patternonto a relatively thick and wide substrate, such as a single crystalsilicon wafer. The ability to use relatively thick X-ray mask substrates(e.g., 300 to 400 μm thick silicon) greatly reduces the cost of maskproduction as compared to the very thin and fragile mask substratesrequired for soft X-ray masks, and allows much wider masks to be made(e.g., 4 to 6 inches in diameter) because of the greater structuralstrength of the thicker substrate, thereby allowing exposure of a widetarget area to increase batch production. The thicker mask is also moredurable and can be used for many more exposures than a thin mask. Theshort wavelength of hard X-rays and collimation of the source also makeit possible to use macroscopically thick self supporting materials as amask. These include conventionally machined self-supporting pieces ofvarious high atomic weight materials including copper, tungsten,molybdenum, tentalum and lead as masks where the precision of the maskis not an issue. However, for a practical, high tolerance, highprecision mask system, particularly where feature sizes may besignificantly less than one mm, a lithographically produced mask, suchas the one to be described, is highly preferred. What must beunderstood, is that the alignment procedure of the present invention isnot limited to the particular type of mask to be described in detailherein. In particular, the present invention is not limited to masksthat have mask membranes as such. Thus, as used herein, the terms deviceand alignment structure "membrane" areas, may be understood to alsodescribe areas corresponding to the device and alignment structure maskpatterns on masks that do not have "membranes".

As illustrated in FIG. 2, formation of an X-ray mask 45 for processingof the first photoresist layer 43 using soft X-rays begins with a cleansilicon wafer 47 to which an approximately 1 micrometer thick opticallytransparent polysilicon (e.g., silicon nitride) layer 49 is applied. Thesilicon nitride layer 49 covers the silicon wafer 47, and may be appliedto the silicon wafer 47 using a conventional vapor deposition technique.

A UV light sensitive photoresist layer 51 is then spun-on to the backside 53 of the X-ray mask 45 over the silicon nitride layer 49, asillustrated in FIG. 3. The spun-on photoresist layer 51 is cured bybaking, e.g., at 90° C.

As illustrated in FIG. 4, the photoresist layer 51 on the back side 53of the X-ray mask 45 is then exposed to UV light 55 through a UV mask57. The UV mask 57 has a non-transparent layer 59 formed thereon in apattern which prevents the passage of the UV light 55 therethrough. Theportions 61 of the photoresist layer 51 that are exposed to the UV light55 through the mask 57 are made susceptible to removal by a developer.

The non-transparent layer 59 is formed on the mask 57 in a pattern thatdefines device and alignment structure membrane areas on the mask 45.Two alternative layouts for the device 63 and alignment structure 65membrane areas on the X-ray mask 45 are illustrated in FIGS. 5 and 6.Each of the alternative membrane layouts illustrated in FIGS. 5 and 6include a single device membrane area 63. As will be described in moredetail below, a mask pattern that defines the microstructure pattern tobe formed on the device substrate 40 will be formed in the devicemembrane area 63 of the X-ray mask 45. The exemplary membrane layout ofFIG. 5 includes two alignment structure membrane areas 65 located onopposite sides of the device membrane area 63. The exemplary membranelayout illustrated in FIG. 6 includes four alignment structure membraneareas 65 also arranged on opposite sides of and around the devicemembrane area 63. As will be discussed in more detail below, a maskpattern defining alignment hole geometries used in the alignmentprocedure of the present invention will be formed in each alignmentstructure membrane area 65 of the X-ray mask wafer 45.

It should be noted that layouts other than those illustrated in FIGS. 5and 6 may also be used in accordance with the present invention. Forexample, 3, 5, or more alignment structure areas 65 may be formed. Thedevice 63 and alignment structure 65 areas may be of various shapes, andarranged in various patterns on the X-ray mask wafer 45. However, inlaying out the alignment structure 65 and device areas 63 on the X-raymask 45, it is preferable to place the alignment structure areas 65 asfar apart as possible on opposite sides of the device area 63. Thislayout will help to minimize alignment inaccuracies. Maximuminaccuracies will typically occur in areas corresponding to thealignment structure areas 65 where, as will be discussed, alignmentposts and alignment holes are joined together. Thus, with, for example,the alignment area layout of FIG. 5, the alignment errors at the centerof the X-ray mask 45, i.e., the area corresponding to the microstructuredevice pattern, will be at most as large as the errors in the alignmentstructure areas 65. For example, if the left side of the X-ray mask 45is shifted downward and the right side is shifted upward when alignedwith the first photoresist layer 43, the same twist angle will appear asa misalignment at the center of the device membrane area. However, theoffset in the center will be smaller than each of the offsets in thealignment structure areas 65. Thus, the alignment structure areas 65 arepreferably chosen to be formed on opposite sides of the device area 63.

Even better alignment accuracy may be achieved with the alignment arealayout illustrated in FIG. 6. Adding additional alignment structureareas 65 on opposite sides of the device area 63 ensures that bothhorizontal and vertical offsets are partially compensated for by reverseshifts at opposite alignment structure areas. For example, in thealignment structure area layout of FIG. 5, if the assembly of thealignment structures is not perfect, e.g., there is a gap between thealignment posts and the alignment holes forming the alignment structuresin the alignment structure areas 65, the X-ray mask 45 can slide by asmall degree in the vertical direction with respect to the photoresistlayer to which it is to be aligned. An error in the horizontal directionis less likely, since the alignment structures formed in the alignmentstructure area 65 on the opposite side of the device area 63 will pullor push in the opposite direction (unless the same gap is found in bothalignment structure areas 65). Thus, the alignment structure area layoutof FIG. 5 tends to center the X-ray mask 45 in a horizontal directionwith respect to the photoresist layer to which it is to be aligned. Thepurpose of the additional alignment areas 65 in the layout of FIG. 6 isto obtain the same effect in the vertical direction. With the layout ofFIG. 6, a smaller and more uniform offset in both the vertical andhorizontal directions can be expected.

After the photoresist layer 51 has been exposed to UV light through theUV mask 57, the exposed portions 61 of the photoresist layer 51 areremoved by developing the photoresist layer 51. As illustrated in FIG.7, this exposes areas 72 of the silicon nitride layer 49. The exposedareas 72 of the silicon nitride layer 49 correspond to the device 63 andalignment structure areas 65 as defined by the pattern of thenon-transparent layer 59 on the UV mask 57.

A reactive ion etch of the exposed areas 72 of the silicon nitride layer49 on the back side 53 of the X-ray mask 45 is then performed. Duringthis operation, the remaining photoresist layer 51 acts as a mask of thesilicon nitride layer 49. Thus, the reactive ion etch removes only theexposed portions 72 of the silicon nitride layer 49. After the reactiveion etch, the remainder of the photoresist layer 51 may be removed. Theresulting structure is illustrated in FIG. 8. As shown, the reactive ionetch leaves windows 74 in the silicon nitride layer 49 on the back side53 of the X-ray mask 45. The windows 74 in the silicon nitride layer 49expose portions of the silicon wafer 47. The exposed portions of thesilicon wafer 47 correspond to the device 63 and alignment structuremembrane 65 areas described previously.

Portions of the silicon wafer 47 exposed by the windows 74 in thesilicon nitride layer 49 are then etched along the silicon crystalboundaries from the back side 53 of the X-ray mask 45 to the front side76 of the X-ray mask 45. This etching process does not attack thesilicon nitride layer 49. The etching process results, when complete, inthe structure illustrated in FIG. 9. The removal of portions of thesilicon wafer 47 by the etching process forms silicon nitride membranes78 and 80 on the front side 76 of the X-ray mask 45. A device membrane78, and alignment structure membranes 80, are thereby formed in thedevice 63 and alignment structure 65 membrane areas defined earlier. Thesilicon nitride device and alignment structure membranes 78 and 80 arethin, fragile and substantially transparent.

In sizing the device 63 and alignment structure 65 membrane area layoutsto be formed on the back side 53 of the X-ray mask 45 which arenecessary to achieve correctly sized device 78 and alignment structure80 membranes on the front side 76 of the X-ray mask 45, it is essentialto consider the important silicon-crystal definition. In the procedurefor forming the device 78 and alignment structure 80 membranes describedabove, etching of the silicon wafer 47 proceeds along silicon-crystalboundaries. Commercially available silicon wafers have pre-defineddirection planes. The windows 74 in the silicon nitride layer 49 on thetop 53 of the X-ray mask 45 are laid out in directions parallel andperpendicular to the planes. The planes in silicon function as an etchstop. Thus, the etch angle a into single crystal silicon can becalculated. From the etch angle, the dimensions of the windows 74 in thesilicon nitride layer 49 on the back side 53 of the X-ray mask 45necessary to obtain a certain size for the silicon nitride membranes 78and 80 on the front side 76 of the X-ray mask 45 can be calculated. Forexample, for commercially available silicon with an etch angle ofα=54.7356°, to obtain a membrane 78 or 80 of specific size, the initialrectangular etch area on the back side 53 of the X-ray mask 45 has to be.increment.x=d/tanα larger in all directions, where d is the thicknessof the silicon wafer 47. For a wafer of approximately 400 micrometers inthickness, .increment.x is about 280 micrometers.

Formation of the X-ray mask 45 proceeds, as illustrated in FIG. 10, withcleaning of the silicon nitride surface on the front side 76 of theX-ray mask 45. A thin metal plating base layer 84 is then applied overthe device and alignment structure membranes 78 and 80. The conductiveplating base layer 84 may be applied, for example, by sputtering a 200angstrom thick layer of titanium and nickel onto the front side 76 ofthe X-ray mask 45. The thin conducting metal plating base layer 84 isslightly transparent to optical light. A photoresist layer 86 is spun-onto the top of the plating base layer 84. The X-ray mask 45 is thenprocessed through a baking cycle to cure the photoresist layer 86.

The cured photoresist layer 86 is exposed to UV light 88 through amaster mask 90 as illustrated in FIG. 11. The master mask 90 includes aglass layer 92 upon which a non-transparent layer 94 is deposited. Thenon-transparent layer 94 is patterned, in this case, with the negativeof desired microstructure device and alignment hole patterns. Thesepatterns may be formed on the master mask 90 in a conventional manner,such as by laser etching of the non-transparent layer 94. Exposure ofthe photoresist layer 86 to UV light through the master mask 90transfers the negative microstructure device and alignment hole patternsto the photoresist layer 86. Exposed portions 96 of the photoresistlayer 86 are susceptible to removal by a developer.

The microstructure device pattern defined by the master mask 90 istransferred onto an area of the photoresist layer 86 corresponding tothe device membrane 78 of the X-ray mask 45. This pattern defines theultimate shape of the microstructure device layer to be formed on thedevice substrate 40. Details of the pattern will, of course, depend onthe nature of the device to be fabricated.

Other portions of the pattern on the master mask 90 define the geometryof first alignment holes to be formed in the first photoresist layer 43on the device substrate 40. These alignment hole patterns aretransferred onto the areas of the photoresist layer 86 corresponding tothe alignment structure membranes 80. Various alignment hole geometriesmay be used. Two possible geometries are illustrated in FIGS. 12 and 13.The alignment structure geometry illustrated in FIG. 13 is designed toprovide many defined borders, to prevent twisting of the alignmentstructures when fully assembled. However, experimentation has shown thatwhen this geometry is used to form alignment holes in the firstphotoresist layer 43 on the device substrate 40, there is an increasedprobability of cracks forming in the photoresist layer 43 at the cornersof the alignment holes after solvent bonding of a second photoresistlayer onto the first photoresist layer 43. These cracks in the area ofthe alignment holes can lead to additional alignment uncertainty.Alignment posts inserted into alignment holes with cracked corners couldslide by an additional amount due to the gaps formed by the cracks. Inorder to minimize cracking of the first photoresist layer 43 attached tothe device substrate 40, the alignment structure geometry illustrated inFIG. 12, with rounded corners, is preferably used for the alignmentholes and posts.

In order to maximize the mechanical stability of the alignment system,the size of alignment structures is preferably maximized. Largealignment structures also ensure an easier assembly of X-ray masks ontoto the first photoresist layer 43 on the device substrate 40. Theexemplary alignment structure geometries illustrated in FIGS. 12 and 13are approximately 5600 micrometers long and 4000 micrometers wide.Differently sized geometries may also be used, of course, it beingunderstood that larger geometries will reduce the usable area of theX-ray mask 45 for the device area 63, and that smaller alignmentstructure geometries will be less stable and more difficult to assemble.In order to maximize available device space, it is preferable that thealignment geometry used nearly fill the entire available area of thealignment structure membranes 80. In other words, the alignmentstructure membranes 80 need not be made significantly larger than thealignment geometry to be employed. A separation between the alignmentstructure geometry and the alignment structure membrane 80 border of 1-2millimeters along the sides, top, and bottom of the membrane 80 issufficient to permit easy alignment of the master mask 90 with thealignment structure membranes 80 of the X-ray mask 45 during the processof making the X-ray mask.

Processing of the X-ray mask 45 continues, as illustrated in FIG. 14, bydeveloping the photoresist layer 86 to remove the exposed portions 96therefrom. This exposes areas 100 of the plating base layer 84 in apattern corresponding to the negative of the device pattern andalignment structure geometries. An X-ray absorbing material, such asgold, is then electroplated onto the plating base layer 84 into theareas 100 where the plating base 84 is exposed. Approximately 3micrometers of gold may be electroplated to form a pattern of X-rayblocking material 102 on the X-ray mask 45. A completed first X-ray mask45 is shown in FIG. 15, after deposition of the X-ray blocking pattern102 and after the remaining portions of the photoresist layer 86 havebeen removed. The X-ray blocking pattern 102 formed on the X-ray mask 45defines the device pattern and alignment structure geometries to beformed on the device substrate 40.

With respect to the foregoing description of an exemplary process forfabricating the first X-ray mask 45, it should be noted that certainsteps, such as cleaning cycles between the steps that are described, andcertain details of other steps, have not been described. However, thesesteps will be widely known to those having skill in the art. The stepsused in forming the X-ray mask 45 are conventional. The formation ofalignment structure membranes and alignment structure geometries on theX-ray mask 45, and their relationship to device membranes andmicrostructure device patterns on the mask 45 are, however, features ofthe present invention.

As illustrated in FIG. 16, the first photoresist layer 43 on the devicesubstrate 40 is exposed to X-rays through the first X-ray mask 45. Thepattern of X-ray absorbing material 102 on the X-ray mask 45 preventsX-rays from impinging on the first photoresist layer 43, except in areas108 corresponding to the microstructure device pattern and areas 106corresponding to alignment hole geometries defined by the X-ray mask 45.Although exposure of the first photoresist layer 43 to X-ray radiationfrom a synchrotron source is preferred, to obtain structures of maximumdepth and minimum run-out, it is apparent that the process may be usedwith other types of radiation, such as ultraviolet (UV) ornon-synchrotron X-rays, where thinner structures are acceptable. (Aphotoresist layer 43 made of a material sensitive to the particular typeof radiation used must be employed.)

The device substrate 40, with patterned first photoresist layer 43, isnow processed to form a first layer of electroplated microstructuredevices on the plating base 41 of the device substrate 40, and to formfirst alignment holes in the photoresist layer 43 that are free from anyelectroplated metal or other processing residues. Before developing thephotoresist layer 43, the exposed portions 106 of the photoresist layer43 corresponding to the first alignment holes are masked, such as with amasking tape 110, as illustrated in FIGS. 17 and 18. The photoresistlayer 43 is then developed in a conventional manner to remove theexposed portions 108 of the photoresist layer 43 corresponding to thefirst layer microstructure device patterns therefrom. As illustrated inFIG. 19, the developing process removes the exposed portions 108 of thephotoresist layer 43, to expose the plating base layer 41 in areas 112corresponding to the first layer microstructure device patterns. Theexposed areas 106 of the photoresist layer 43 corresponding to the firstalignment holes to be formed in the photoresist layer 43, however,remain intact. Metal microstructures 114 are then electroplated onto theplating base layer 41 in the areas 112 from which the photoresist layer43 was removed by the developing process. The resulting structure isillustrated in FIG. 20, after the masking tape 110 has been removed.

To fabricate a multi-layer metal microstructure using the LIGA process,a second metal microstructure layer will be deposited on the first layermetal microstructures 114. This requires that the upper surface of thefirst layer microstructures 114 be maximally flat. However, theelectroplated microstructures 114 formed by the electroplating processwill not typically have a flat surface. The surface of the electroplatedstructures 114 will tend to be concave in shape, with more materialdeposited around the walls of the wells formed in the first photoresistlayer 43. A second concern is that the height of the electroplatedmicrostructures 114 of the first layer be as nominally designed. It isdifficult to control the electroplating process to provide electroplatedstructures 114 of the exact height desired. It is also preferable thatthe top surface of the first photoresist layer 43 itself be mademaximally flat, to assure proper adhesion of a second photoresist layeron top of the first photoresist layer 43. Thus, to assure properadhesion between the first layer microstructures 114 and firstphotoresist layer 43 and subsequent microstructure and photoresistlayers, a replanarization process is preferably employed to form amaximally flat surface on the first layer of microstructures 114 andphotoresist 43, and to reduce the first layer microstructures 114 totheir designed height. Preferably, the surface of the first photoresistlayer 43 and microstructures 114 is also polished to an optical finishby the replanarization step.

Replanarization of the surface of the first photoresist layer 43 andfirst layer microstructures 114 may be accomplished in a conventionalmanner, for example, by micromilling of the photoresist 43 andmicrostructure 114 surfaces. Flat photoresist 43 and microstructure 114surfaces, at the desired microstructure height, may be obtained in thismanner. However, conventional replanarization methods typically exertundesirable sheer forces on the photoresist layer 43 and microstructures114. Thus, especially for tall narrow microstructure shapes, adhesionbetween the microstructures 114 and the plating base 41 may be lostduring conventional replanarization.

A preferable method for replanarizing the photoresist layer 43 andmicrostructures 114 is a lapping process using a diamond containinglapping slurry. A lapping machine is furnished with a lapping plate madeof a soft metal material. The lapping plate surface is furnished withridges of controlled height using a diamond embedded conditioning ringwith a specified grit size. Free diamonds in a liquid slurry are thensprayed onto the plate and embedded therein by a ceramic conditioningring. After the lapping plate is conditioned, the surface of thephotoresist layer 43 and first layer microstructures 114 is mountedagainst the lapping plate. The lapping plate is then rotated against thephotoresist and microstructure layer surface. During the lappingprocess, additional diamond slurry is sprayed onto the lapping plate,and driven into the plate by the ceramic conditioning ring. The size ofdiamonds in the diamond lapping slurry is selected to control the sheerforces applied to the surface being lapped, and to achieve a desiredsurface finish. In this manner, material is removed from the surface ofthe first photoresist layer 43 and first metal microstructure layer 114to planarize the surface and reduce the first layer microstructures 114to their designed height. A polishing step, using a cloth covered hardmetal polishing plate and loose diamond slurry, is preferably employedafter lapping to provide a smooth optical surface finish to the firstmicrostructure layer 114. This lapping and polishing process preparesthe first photoresist layer 43 for adhesion of a second photoresistlayer thereto, and prepares the first microstructure layer 114 forelectroplating of a second microstructure layer thereon, whileminimizing the risk of damage to either the first photoresist layer 43or the first microstructure layer 114 due to excessive sheer forces.

After the surface of the first photoresist layer 43 and firstmicrostructure layer 114 is replanarized, the first photoresist layer 43is developed for a second time to remove the exposed portions 106therefrom to form the first alignment holes 116. The resulting devicesubstrate structure is illustrated in FIG. 21.

Variations on the described procedure for forming the device substratestructure illustrated in FIG. 21 may also be employed in accordance withthe present invention. For example, the exposed areas 106 and 108 of thefirst photoresist layer 43, corresponding to first alignment holegeometries and first microstructure device patterns, respectively, maybe removed at the same time by developing the first photoresist layer 43in a single step. After development of the first photoresist layer 43,masking tape is placed over the first alignment holes 116.Electroplating of the first layer metal microstructures 114 thenproceeds as described above. In this case, the masking tape prevents thealignment holes 116 from filling with metal during the electroplatingprocess. The masking tape is then removed for replanarization of themetal microstructures 114. However, if the preferred lapping andpolishing method described previously is used for replanarization, thediamond lapping and polishing slurry will inevitably fill into thealignment hole cavities 116. Experiments have shown that this slurrycannot be easily removed from the alignment holes 116 by scratching,flushing, vacuum suction, solvents, or detergents. The presence ofslurry in the alignment holes makes assembly of the alignment structuresof the present invention, which requires the insertion of alignmentposts into the alignment holes 116, virtually impossible. A solution tothis problem is to seal the first alignment holes 116 afterelectroplating, but before diamond slurry lapping, with exposed orunexposed photoresist. However, this typically leads to a compound ofphotoresist and diamond lapping slurry being embedded into the firstalignment holes 116. This compound also cannot easily and completely beremoved from the alignment holes 116 by conventional processingtechniques, e.g., by using isopropanol.

After the first photoresist layer 43 and microstructure layer 114 arereplanarized, and thoroughly cleaned, a second photoresist layer 118 isapplied onto the first photoresist layer 43. As illustrated in FIG. 22,the second photoresist layer 118 is preferably formed from a preformedphotoresist sheet of desired thickness that is solvent bonded onto thefirst photoresist layer 43 over the first layer of microstructures 114.As described previously, solvent bonding may preferably be accomplishedby spinning a thin layer of liquid PMMA onto the upper surface of thefirst photoresist layer 43 and first layer microstructures 114. Aphotoresist sheet is then applied over this spun-on layer of liquidphotoresist to form the second photoresist layer 118. During spinning onof the thin liquid photoresist layer, the alignment holes 116 arepreferably masked, such as with masking tape in the manner describedpreviously with respect to FIGS. 17 and 18, to prevent the firstalignment holes 116 from filling with the liquid photoresist. The sizeof the area around the first alignment holes 116 that should be maskedduring application of the liquid photoresist layer is preferably atleast as large as the size of photoresist spacer sheets that may be usedfor assembling a second X-ray mask onto the first photoresist layer 43.The use of photoresist spacer sheets in the method of the presentinvention will be described in more detail below.

It should be apparent that the size of the photoresist sheet used toform the second photoresist layer 118 is smaller than that of thephotoresist sheet used to form the first photoresist layer 43. Thesecond photoresist layer 118 must, of course, not cover the firstalignment holes 116. In addition, the size of the second photoresistsheet must provide sufficient space between the second photoresist layer118 and the alignment holes 116 to accommodate mask photoresist layersattached to the second X-ray mask which, as described in more detailbelow, contain mask alignment holes that are aligned with the firstalignment holes 116 in the first photoresist sheet 43. Thus, forexample, for a first photoresist layer sheet size of 63×35 millimeters,the photoresist sheet size for the second photoresist layer 118 may beselected to be 32×30 millimeters.

With the second photoresist layer 118 applied to the first photoresistlayer 43, the device substrate is prepared for the formation of a secondlayer of microstructure devices. The first step in the formation of asecond microstructure device layer is patterning of the secondphotoresist layer 118. The second layer microstructure device pattern isdefined by a pattern of X-ray blocking material on a second X-ray mask.The second X-ray mask will be fabricated in essentially the same manneras the first X-ray mask. Thus, for hard X-ray exposures, the mask may beformed of microstructure device and alignment hole geometry maskpatterns formed on device and alignment structure areas, respectively,of a thick silicon substrate. We will continue, however, with theexemplary use of a soft X-ray mask. The fabrication of the second softX-ray mask may be accomplished in the same manner as describedpreviously with respect to the first X-ray mask. Thus, a silicon wafercovered by a layer of silicon nitride is patterned and etched to formdevice and alignment structure membranes. The same layout of device andalignment structure areas as was used in the fabrication of the firstX-ray mask is used for the fabrication of the second X-ray mask. Amaster mask is then used to form a pattern of X-ray blocking material onthe second X-ray mask in the membrane areas. The mask pattern in thedevice membrane area of the second X-ray mask defines the second layermicrostructure device pattern to be formed. The mask patterns in thealignment structure membrane areas of the second X-ray mask defineexactly the same geometries as were used to form the first alignmentholes 116 in the first photoresist layer 43. Since the masking andexposure processes used to form the microstructure device patterns andthe alignment hole geometries on the second X-ray mask are carried outsimultaneously, the relative positions of the alignment hole geometrieswith respect to the microstructure device patterns is preciselycontrolled. This relationship is defined by the master mask used topattern the second X-ray mask such that, when the alignment holegeometries defined by the second X-ray mask are aligned with thealignment hole geometries defined by the first X-ray mask (which wereused to form the first alignment holes 116), the microstructure devicepatterns defined by the first and second X-ray masks will also bealigned.

As illustrated in FIG. 23, the second X-ray mask 120 includes X-rayabsorbing material 122 formed thereon in a pattern which defines boththe second layer microstructure device pattern and alignment holegeometries for the second X-ray mask 120. In accordance with the presentinvention, the second X-ray mask 120 is aligned with the firstphotoresist layer 43 on the device substrate 40 by alignment posts usedto align the alignment holes 116 in the first photoresist layer 43 withmask alignment holes formed in mask photoresist layers attached to thesecond X-ray mask 120. The mask alignment holes in the mask photoresistlayers are formed by first attaching photoresist sheets to the secondX-ray mask 120 in positions whereby the mask photoresist layers may beexposed to X-rays through the alignment hole geometry pattern defined bythe second X-ray mask 120.

There are several alternative procedures for attaching mask photoresistlayers to the second X-ray mask 120. A first alternative, as illustratedin FIG. 23, is to glue photoresist sheets 124 directly onto thealignment structure areas of the mask. In this case, care must be takento ensure that the alignment structure membranes 126 do not crack due tothe glue-down procedure. Also, care must be taken to prevent breaking ofthe alignment structure membranes 126 during the process of exposing themask photoresist layers 124 to X-rays through the X-ray mask 120.Bubbling of the photoresist layer 124 during X-ray exposure may crackthe alignment structure membrane 126. Of course, the problem is avoidedfor hard X-ray masks that do not require thin and fragile membranes inthe patterned areas thereof to allow X-rays to pass through the mask.

A second alternative procedure for attaching the mask photoresist layers124 to the second X-ray mask 120 is to glue photoresist sheets 124 tothe back side of the X-ray mask 120, as illustrated in FIG. 24. Asshown, the photoresist sheets 124 are attached to the back side of thesecond X-ray mask 120 in alignment with the alignment membranes 126.This procedure involves some complex steps, and a more complicated X-raymask fabrication sequence than other available procedures. Since X-rayexposures are performed in a vacuum, air captured in the spaces 128between the alignment structure membranes 126 and mask photoresistlayers 124 could expand during the X-ray exposure process and break thealignment structure membranes 126. Therefore, a hole would need to bedrilled into the center of the mask photoresist layers 124, in an areaof the photoresist sheets 124 which is to be exposed to X-rays andremoved by development, before the photoresist sheets 124 are glued downto the X-ray mask 120, to enable air in the cavities 128 to escape.After developing, the removal of developer that seeps into the cavities128 between the mask photoresist sheets 124 and the alignment structuremembranes 126 could be difficult. Furthermore, this procedure introducesan additional gap between the mask photoresist layers 124 and thepatterned alignment structure membranes 126. Although synchrotron X-raysprovide highly collimated exposures, there will be some X-ray exposurerun-out over the approximately 400 micrometer distance typicallyseparating the mask photoresist layers 124 and alignment structuremembranes 126. Thus, the mask alignment holes formed in the maskphotoresist layers 124 attached to the second X-ray mask 120 will beslightly larger than the alignment hole geometries defined by the maskpattern. This will result in decreased alignment accuracy unless thesize of the alignment hole geometries defined by the mask pattern isadjusted to compensate for this run-out effect. Finally, if the maskphotoresist layer 124 is attached to the back side of the X-ray mask120, the X-ray mask 120 must be used "upside down" to align the secondX-ray mask 120 with the first photoresist layer 43 on the devicesubstrate 40. Thus, the layout of the second x-ray mask pattern will bethe mirror image of the mask pattern which would normally be used if themask photoresist layers 124 were attached to the front side of thesecond X-ray mask 120. Some of these problems may be avoided for hardX-ray masks, which may be formed on solid thick silicon wafers, andwhich do not have thin membranes or cavity structures.

The preferred procedure for attaching the mask photoresist layers 124 tothe X-ray mask 120 is illustrated in FIGS. 25 and 26. In thisembodiment, the mask photoresist layers 124 are attached to the frontside of the X-ray mask such that they are elevated over the alignmentstructure membranes 126. This is achieved, as illustrated best in FIG.26, by gluing rectangular photoresist sheets 128 and 129 to the frontside of the second X-ray mask 120, outside of the alignment structureareas 126. Then, longer sheets of photoresist 124, of approximately thesame width as the first sheets of photoresist 128 and 129, are glued tothe sheets 128 and 129 which are attached to the second X-ray mask 120to form the mask photoresist layers 124. The mask photoresist layers 124thus extend over the alignment structure membrane areas 126 on the frontside of the X-ray mask and are separated therefrom. This preferredprocedure does require additional photoresist cutting and solventbonding steps.

The mask photoresist layers 124 attached to the second X-ray mask 120are exposed to X-rays through the X-ray mask pattern. This causes aportion of the mask photoresist layers 124, corresponding to the maskalignment hole geometries defined by the X-ray mask pattern, to becomesusceptible to removal by a developer. During the development step, therelatively large gap between the mask photoresist layer 124 and thealignment structure membranes 126 provides for a rather large flow ofdeveloper between the photoresist layers 124 and the membranes 126. Asillustrated in FIG. 27, exposure and development of the mask photoresistlayers 124 form mask alignment holes 130 in the mask photoresist layers124 attached to the second X-ray mask 120. The mask alignment holes 130in the mask photoresist layers 124 on the second X-ray mask 120 have thesame geometry as the first alignment holes 116 in the first photoresistlayer 43 on the device substrate 40.

In accordance with the present invention, alignment of the second X-raymask 120 with the first photoresist layer 43 is achieved by assemblingthe second X-ray mask 120 onto the first photoresist layer 43 usingalignment posts to connect the first alignment holes 116 on the firstphotoresist layer 43 with the mask alignment holes 130 in the maskphotoresist layer 124 attached to the second X-ray mask 120. Thealignment posts, therefore, must have the same geometry as the alignmentholes 116 and 130. The alignment posts may be made of metal using aconventional LIGA fabrication process employing a sacrificial layer tofree electroplated metal alignment posts from the substrate on whichthey are formed. Alternatively, and preferably, the alignment posts maybe fabricated of a photoresist material, such as PMMA. An exemplaryprocedure for fabricating photoresist alignment posts is described withreference to FIGS. 28-31.

Referring initially to FIG. 28, the process of fabricating a photoresistalignment post begins with the application of a thin sacrificial layer140 onto a substrate 141. The substrate 141 typically has a planarsurface on which the sacrificial layer 140 is formed, and may be made ofa variety of materials, e.g., silicon wafer, glass, metal, or variousplastics. The material of the sacrificial layer 140 may be any of avariety of materials which is resistant to attack from a photoresistdeveloper. For example, where the photoresist is PMMA, the sacrificiallayer of material must be resistant to a typical PMMA developer such asmorpholine, 2-(2-butoxyethoxy) ethanol, ethanolamine, and water. Thesacrificial layer of material 140 must also be selectively removable bya remover which does not attack the photoresist, e.g., PMMA. For a PMMAphotoresist, examples of suitable sacrificial layers are titanium(sputtered onto the substrate), which can be removed with dilutehydrofluoric acid, and partially imidized polyimide (which is spun onthe substrate), with a suitable remover for the polyimide being ammoniumhydroxide. A soft PIRL polyimide material can also be used as thesacrificial layer. A preformed strain free photoresist sheet ispreferably adhered to the sacrificial layer 140, such as by solventbonding to form a photoresist layer 142. The photoresist layer 142 maybe milled down, if necessary, to reach a desired alignment post heightthat is less than the initial photoresist sheet thickness.

As illustrated in FIG. 29, an X-ray mask 144, having X-ray absorbingpatterns 145 formed thereon, provides a pattern exposure fromsynchrotron radiation X-rays 146 to provide exposed patterns 147 in thephotoresist layer 142. The X-ray mask pattern 145 defines the geometryof the alignment posts, e.g., one of the geometries illustrated in FIG.12 or FIG. 13, which is identical to the geometry of the alignment holes116 and 130 formed in the first photoresist layer 43 on the devicesubstrate 40 and the mask photoresist layers 124 on the second X-raymask 120, respectively. The exposed photoresist 147 is then developedusing a high selectivity developer, as described above, to remove theexposed photoresist 147, leaving, as illustrated in FIG. 30, isolatedalignment post structures 148 adhered to the sacrificial layer 140. Aremover of the sacrificial layer 140 is then applied to the sacrificiallayer 140 on the substrate 141 to selectively etch away the sacrificiallayer 140, thereby freeing the alignment posts 148 from the substrate.

An exemplary free alignment post 148 formed by the foregoing process,and having an alignment post geometry approximately corresponding tothat illustrated in FIG. 12, is illustrated in FIG. 31. Because asubstantially strain free preformed photoresist sheet is preferably usedto form the alignment posts 148, the alignment posts 148 will havesubstantially no internal strain when freed from the substrate 141, andtherefore will not substantially mechanically distort. This is desirablefor maintaining alignment accuracy between the second X-ray mask 120 andthe first photoresist layer 43 on the device substrate 40 using thealignment posts 148.

Another process which may be used to fabricate photoresist alignmentposts 148 is to expose a free preformed PMMA sheet, i.e., a PMMA sheetnot attached to a substrate, to X-rays through an X-ray mask thatdefines the alignment post geometries. Development of the exposedphotoresist sheet then automatically produces the alignment posts 148.This process eliminates the need for applying a photoresist sheet to asubstrate and the steps of etching or dissolving a sacrificial layer tofree the formed alignment posts 148 from the substrate. However, sincethe photoresist alignment posts 148 are not attached to a substrate, andare automatically freed by development of the photoresist sheet, theymay easily become lost during the development cycle. Hence, precautions,such as use of a sieve, are needed for the use of this procedure.

The second X-ray mask 120 is aligned with the first photoresist layer 43on the device substrate 40 by assembling the second X-ray mask 120 ontothe first photoresist layer 43 using the alignment posts 148, asillustrated in FIG. 32. No specialized equipment is required to achievealignment between the second X-ray mask 120 and the first photoresistlayer 43. Accurate alignment automatically results from assembly of thesecond X-ray mask 120 onto the first photoresist layer 43 using thealignment posts 148. Assembly can be easily accomplished within a fewminutes at the X-ray exposure site.

The steps of the assembly procedure are as follows. First, alignmentposts 148 are placed into each of the alignment holes 116 in the firstphotoresist layer 43 on the device substrate 40. The alignment posts 148may be placed into the alignment holes 116 using a tweezers. Thealignment posts 148 extend from the alignment holes 116 above thesurface of the first photoresist layer 43 on the device substrate 40.The second X-ray mask 120 is then carefully placed over the firstphotoresist layer 43 on the device substrate 40. Contact of partsextending from the first photoresist layer 43, including the alignmentposts 148 and second photoresist layer 118, with the second X-ray maskdevice membrane, must be avoided. The X-ray mask 120 is then slid untilthe mask alignment holes 130 are over the alignment posts 148 and thealignment posts 148 penetrate into the mask alignment holes 130. Duringthis assembly process, the alignment structure membranes 126 in thesecond X-ray mask 120 might break. However, only the device membrane inthe second X-ray mask 120 is needed for exposure of the secondphotoresist layer 118. The mask alignment holes 130 in the maskphotoresist layers 124 on the X-ray mask 120 already incorporate thedesired alignment hole geometry. Hence, the alignment structuremembranes 126 may be broken on purpose to allow the mask alignment holes130 and the alignment posts 148 to be seen clearly during the assemblyprocess.

Alignment of the second X-ray mask 120 with the first photoresist layer43 by the assembly procedure of the present invention results inaccurate alignment between the microstructure device mask pattern on thesecond X-ray mask 120 and the microstructure device pattern on the firstphotoresist layer 43. Factors which can adversely affect alignmentaccuracy, and methods of compensating for and/or minimizing the effectof these factors, will now be discussed briefly.

During X-ray exposure of a photoresist layer, beam divergence occurs,which leads to slight run-outs in the sidewalls of the structures beingproduced. For synchrotron source X-rays, run-outs are typically on theorder of 0.1 micrometers per 100 micrometers of X-ray exposure depth. Inthe process of fabricating the alignment structures of the presentinvention, run-outs occur at three different steps: exposure of thefirst photoresist layer 43 on the device substrate 40, exposure of themask photoresist layers 124 attached to the second X-ray mask 120, andin the fabrication of the alignment posts 148. There are two differentcategories of run-outs: runouts within a single photoresist layer, andrun-outs between an X-ray mask and a photoresist layer due to theexposure gap between the mask and the photoresist layer. Run-outs due tothe exposure gap can be compensated for by a mask pattern which isadjusted in size laterally by the gap times the run-out. This adjustmentof the lateral size of the mask pattern will result in an exposure ofthe photoresist layer across the exposure gap which corresponds to thedesired nominal size of the structure to be produced. Run-outs within aphotoresist layer itself, however, cannot be compensated. The effect ofthese run-outs on the alignment structure of the present invention isillustrated schematically in FIG. 33. This figure shows a schematiccross-sectional view of an alignment post 148 assembled in the alignmentholes 116 and 130 in the first photoresist layer 43 on the devicesubstrate 40 and in a mask photoresist layer 124 on the X-ray mask 120,respectively. The run-out angles shown in FIG. 33 are exaggerated forpurposes of illustration. Ideally, the alignment post 148 is laid out tofit exactly into the first alignment hole 116 in the first photoresistlayer 43 on the device substrate 40. However, due to X-ray exposurerun-out, this is difficult to achieve. This results in a small gap 150between the alignment post 148 and the walls of the first alignment hole116. A small gap may also remain between the walls of the mask alignmenthole 130 and the alignment post 148. The effect of the gap 150 betweenthe first photoresist layer 43 on the device substrate 40 and thealignment post 148 can be calculated, ignoring all other effects.Assuming a 150 micrometer thick first photoresist layer 43, the lateralrun-out resulting from synchrotron source X-ray exposure isapproximately 0.15 micrometers. X-ray exposure run-out within the firstphotoresist layer 43 is, therefore, certainly not a main contributor toalignment errors. Hence, this is not a main area of concern. However, itis important to keep the effect of X-ray exposure run-out over theexposure gap in mind. Run-outs from a 400 micrometer gap are in the 0.5micrometer range. This, combined with other effects, represents a moresignificant influence on alignment accuracy. However, as discussedpreviously, the effect of the exposure gap can be compensated for duringlayout of the X-ray mask patterns. Also, since run-outs and processingvariables reduce the effective post size 148, a small overdimensioningof the posts 148 leads to a better press-fit assembly of the secondX-ray mask 120 onto the first photoresist layer 43, and hence to abetter alignment accuracy.

Another potential cause of alignment inaccuracy is temperature variationbetween different steps of the alignment structure fabricationprocedure. For example, if the temperature during exposure of the firstphotoresist layer 43 on the device substrate 40 is different from thetemperature during exposure of the mask photoresist layer 124 on thesecond X-ray mask 120, a shift of alignment structures could occur. Thisis caused by thermal expansion or contraction of either the mask or thesubstrate. This results in different relative positions of the alignmentholes 130 and 116 on the mask and substrate. This misalignment couldmake it difficult to assemble the second X-ray mask 120 onto the firstphotoresist layer 43. One way to deal with these unexpected processinginaccuracies is to fabricate multiple alignment posts 148 having thesame geometry but of slightly different sizes. For example, alignmentposts that are 0, 1, 2, and 4 microns smaller than the desired size ofthe alignment holes 116 and 130 may be produced. (To distinguish betweenthe different sizes of posts, holes may be added to the posts 148 foridentification purposes.) one of these alignment post sizes should allowfor assembly of the second X-ray mask 120 onto the first photoresistlayer 43, without dramatically affecting the alignment accuracy.However, the best solution is to minimize unexpected processinginaccuracies, by maintaining constant temperatures through each of thephotoresist exposure steps described.

Referring once again to FIG. 32, the X-ray mask 120 and device substrate40 may be secured together, after assembly using the alignment posts148, by taping the mask 120 to the device substrate 40. The assembly ofsecond X-ray mask 120 and device substrate 40 is then clamped, in aconventional manner, for exposure of the second photoresist layer 118 toX-rays 152 through the second X-ray mask 120. (To ensure that the X-raymask 120 does not crack from the clamping force, two spacer sheets (notshown) may be placed between the substrate 40 and the X-ray mask 120outside of the device and alignment structure membrane areas.) Beforeexposure of the second photoresist layer 118 to X-rays 152, thealignment structure membrane areas on the second X-ray mask 120, whichmay now be broken, are preferably masked, such as with masking tape orresist 154, to prevent the alignment posts 148 from being exposed toX-rays and from falling out of the alignment holes 116 and 130 throughthe X-ray mask 120 during X-ray exposure. During X-ray exposure, thescanning width of the X-ray beam may also preferably be adjusted so thatonly the photoresist under the device membrane of the second X-ray mask120 is exposed.

Exposure of the second photoresist layer 118 to X-rays 152 through thesecond X-ray mask 120 transfers the second layer microstructure devicepattern from the X-ray mask 120 to the second photoresist layer 118. Forelectroplating of second layer microstructures, the exposed portions 156of the second photoresist layer 118 must correspond to previouslydeposited metal microstructures 114 in the first layer of photoresist43, which are used as a plating base for the second microstructurelayer. Alternatively, X-ray exposure through the second X-ray mask 120may be allowed to proceed for a sufficient duration to expose throughboth the second 118 and first 43 photoresist layers on the devicesubstrate 40. This will permit metal microstructures to be electroplateddirectly onto the plating base layer 41.

In the alignment structure assembly of FIG. 32, the thickness of thesecond photoresist layer 118 on the device substrate 40 is limited to alittle less than the total height of the photoresist sheets 124 and128/129 (see FIG. 26) attached to the second X-ray mask 120. If thesecond photoresist layer 118 is almost as thick as the total height ofthe photoresist sheets 124 and 128/129 on the second X-ray mask 120, theexposure dose needs to be accurately adjusted. This is because anexposure dose that leads to overexposure and bubbling of the secondphotoresist layer 118 may crack the device membrane of the second X-raymask 120. To accommodate different thicknesses for the secondphotoresist layer 118, there is some flexibility in the thickness of thephotoresist sheets 124 and 128/129 that may be attached to the secondX-ray mask 120. Thicker photoresist sheets 124 and 128/129 attached tothe second X-ray mask 120 enable larger second photoresist layers 118 tobe used on the device substrate 40.

To extend the alignment procedure of the present invention toaccommodate larger exposure gaps between the second X-ray mask 120 andthe second photoresist layer 118, thicker second photoresist layers 118,or more than one additional photoresist layer on the device substrate40, spacer sheets 158 may be used, as illustrated in FIG. 34. The spacersheets 158 may be made out of photoresist, such as PMMA, in the samemanner as the alignment posts 148. The spacer sheets 158 include centralapertures which correspond in geometry to that of the alignment holes116 and 130 in the first photoresist layer 43 on the device substrate 40and in the mask photoresist layer 124 attached to the second X-ray mask120. Thus, the central apertures of the spacer sheets 158 are designedto accommodate the alignment posts 148.

The steps for assembly of the structure illustrated in FIG. 34 are asfollows. First, alignment posts 148 are placed into each of thealignment holes 116 in the first photoresist layer 43 on the devicesubstrate 40. Spacer sheets 158 are then placed onto the alignment posts148, which extend from the surface of the first photoresist layer 43,such that the alignment posts 148 extend into the central apertures ofthe spacer sheets 158. Additional alignment posts 160 are then placedinto the remaining space in the central apertures of the spacer sheets158. The second X-ray mask 120 is then carefully placed over the devicesubstrate 40 until the second alignment posts 160, extending from thespacer sheets 158, penetrate into the alignment holes 130 on the secondX-ray mask 120. Securing of the X-ray mask 120 to the device substrate40, and exposure of the second photoresist layer 118 to X-rays 152through the X-ray mask 120, then proceed as described above with respectto FIG. 32. It is apparent that the use of multiple spacer sheets 158,or thicker spacer sheets 158, will allow the alignment procedure of thepresent invention to be extended to accommodate almost any exposure gap,second photoresist layer thickness, or number of additional photoresistlayers attached to the device substrate 40 desired. To provideadditional stability and alignment accuracy to the structure illustratedin FIG. 34, a single longer alignment post may be used in place of thetwo stacked alignment posts 148 and 160.

After the second photoresist layer 118 has been exposed to X-rays 152through the second X-ray mask 120, the exposed portions 156 of thesecond photoresist layer 118 (and, if appropriate, of the firstphotoresist layer 43) are removed by a development process as describedpreviously. The resulting structure is illustrated in FIG. 35. As shown,the development process exposes the first layer metal microstructures114 (and plating base layer 41) in areas 162 onto which the second layermetal microstructures are to be electroplated. During the subsequentelectroplating step, the first alignment holes 116 in the firstphotoresist layer 43 are preferably masked with tape 164, to preventmetal microstructures from being formed therein. This preserves thefirst alignment holes 116, allowing third and subsequent X-ray masks tobe aligned with the first photoresist layer 43 by alignment postsextending between the first alignment holes 116 in the first photoresistlayer 43 and corresponding mask alignment holes formed in maskphotoresist layers attached to the subsequent X-ray masks in the mannerdescribed previously with respect to the second X-ray mask 120. Thefirst alignment holes 116 should also be covered to keep them free fromany debris resulting from the replanarization of the second orsubsequent microstructure layers.

The first and second photoresist layers 43 and 118 may be removed afterelectroplating of the second layer metal microstructures 166. Thisresults, after clean-up, in the structure illustrated in FIG. 36,wherein free standing multi-layer metal microstructures are formed onthe device substrate 40. Freed multi-layer microstructures may beproduced by use of a sacrificial layer between the plating base 41 andsubstrate 40 which, at this point, would be etched away to free themulti-layer microstructures from the substrate 40.

The accuracy of the alignment procedure of the present invention hasbeen verified by the experimental fabrication of multi-layer metalmicrostructures by the LIGA fabrication process and employing thealignment procedure. Alignment accuracies of significantly better than 4microns have been achieved. Alignment accuracies in the sub-micron rangeare achievable using the alignment procedure of the present invention bybetter controlling the alignment accuracy variables describedpreviously, such as temperature variations and X-ray exposure run-out.Since no specialized alignment equipment is required for the alignmentprocedure of the present invention, the present invention provides a lowcost procedure for achieving highly accurate alignment of X-ray maskswith device substrates.

It is apparent that the present invention is not limited to theexemplary application of the fabrication of multi-layer metalmicrostructures using the LIGA process as described in detail herein.The alignment procedure of the present invention may be employedwherever the alignment of multiple masks with a substrate is required.Thus, the present invention may be used for the accurate fabrication ofmulti-layer metal microstructures, photoresist microstructures, orphotoresist laminates having microstructures formed therein. Inaddition, the alignment procedure of the present invention may be usedin the fabrication of single layer microstructures of metal orphotoresist wherein multiple exposures of a single photoresist layerthrough multiple masks are required. Where hard X-ray exposures areemployed, microstructure devices may be fabricated in accordance withthe present invention by aligning the X-ray mask using alignment holesin a first photoresist sheet attached to one side of a device substrate,and then exposing through the substrate to pattern a photoresist layer,or a series of photoresist layers, attached to the side of the devicesubstrate opposite the mask. The alignment procedure of the presentinvention is also applicable to integrated circuit fabrication and,particularly, is compatible with the fabrication of microstructures onsubstrates upon which integrated circuits have previously been formed.

It is thus understood that the present invention is not confined to theparticular embodiments and exemplary applications set forth herein asillustrative, but embraces such modified forms thereof as come withinthe scope of the following claims.

What is claimed is:
 1. A radiation mask adapted to be aligned with aphotoresist layer, comprising:(a) a device area made of a radiationtransmitting material; (b) radiation blocking material formed in thedevice area of the mask in a pattern defining a microstructure device;(c) an alignment structure area; (d) radiation blocking material formedin the alignment structure area in a pattern defining a mask alignmenthole geometry; and (e) a mask photoresist layer attached to the maskover the alignment structure area and having mask alignment holes formedtherein.
 2. The radiation mask of claim 1 wherein the radiation blockingmaterial is an X-ray absorbing material.
 3. The radiation mask of claim2 wherein the radiation transmitting material includes a silicon nitridemembrane and the X-ray absorbing material is gold.
 4. The radiation maskof claim 1 wherein the mask photoresist layer is elevated over thealignment structure area.
 5. The radiation mask of claim 1 includingadditionally a first photoresist layer having first alignment holesformed therein, and alignment posts inserted into the first alignmentholes and mask alignment holes to join the mask and the firstphotoresist layer to form a radiation mask assembly.
 6. The radiationmask of claim 1 wherein there are at least two alignment structure areaspositioned on the mask on opposite sides of the device area.
 7. A methodfor making a radiation mask adapted to be aligned with a photoresistlayer, comprising the steps of:(a) forming device and alignmentstructure areas made of a radiation transmitting material on a maskwafer; (b) forming in the device area a pattern of radiation blockingmaterial defining a microstructure device and in the alignment structurearea a pattern of radiation blocking material defining an alignment holegeometry; (c) attaching a mask photoresist layer to the mask over thealignment structure area; (d) exposing the mask photoresist layer toradiation through the pattern of radiation blocking material in thealignment structure area to make a selected portion of the maskphotoresist layer corresponding to the alignment hole geometry subjectto removal by a developer; and (e) developing the exposed maskphotoresist layer to remove the selected portion thereof to form maskalignment holes therein.
 8. The method of claim 7 wherein the radiationblocking material is an X-ray absorbing material and wherein the step ofexposing the mask photoresist layer to radiation includes the step ofexposing the mask photoresist layer to X-ray radiation.
 9. The method ofclaim 7 wherein the step of attaching a mask photoresist layer to themask includes the steps of attaching photoresist sheets to the maskoutside of alignment structure membrane areas and attaching the maskphotoresist layer to the photoresist sheets such that the maskphotoresist layer is attached over the alignment structure membrane areaand is separated from an alignment structure membrane.